KR20060048364A - Steel excellent in corrosion-resistance, for shipbuilding - Google Patents

Abstract

Corrosion resistance of shipbuilding steels that can be put into practical use without painting or electrical methods, especially for crevice corrosion, and at the same time, corrosion of salts caused by seawater and corrosion due to wet environment To provide shipbuilding steels with excellent durability.

Corrosion-resistant steel for shipbuilding of the present invention contains C: 0.01 to 0.30%, Si: 0.01 to 1.50%, Mn: 0.01 to 2.0%, Al: 0.005 to 0.10%, in addition to Co: 0.01 to 5.00% and Mg : 0.0005 ~ 0.020%, and remainder consists of iron (Fe) and an unavoidable impurity.

Description

Steel material for corrosion resistance {Steel excellent in corrosion-resistance, for shipbuilding}

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the external shape of the test piece A used for the corrosion resistance test.

2 is a view showing the external appearance of the test piece B used in the corrosion resistance test.

3 is a view showing the external appearance of the test piece C used in the corrosion resistance test.

4 is a view showing the external appearance of the test piece D used in the corrosion resistance test.

5 is a view showing the external appearance of the test piece E used in the corrosion resistance test.

Fig. 6 is a diagram showing the appearance of the specimen F used in the corrosion resistance test.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to corrosion resistant steel for ships, which is used as an important structural material in ships such as oil tankers, cargo ships, ships, passenger ships, warships, and the like. The present invention relates to marine steels having excellent corrosion resistance under an environment.

Steel materials used as the main structural materials (for example, outer shells, ballast tanks, tankers, etc.) of the above-mentioned various vessels often suffered corrosion damage from salts caused by seawater or a constant temperature and humidity environment. Such corrosion has a risk of causing sea accidents such as immersion and sinking, and therefore, it is necessary to take some anticorrosive means for the steel. As the conventional anticorrosive means, (a) coating and (b) electric anticorrosion are well known in the related art.

Among the coatings represented by the middle coating, there is a high possibility of coating defects, and there may be a defect in the coating due to a collision in the manufacturing process or the like. ) Is often exposed. Such steel exposed portions are corroded locally or intensively, leading to premature leakage of petroleum-based liquid fuel contained therein.

On the other hand, the electric method is a very effective means for the part completely immersed in seawater, but the electric circuit necessary for the method is not formed in the part receiving the seawater spray in the air, so There are times when it is not enough. In addition, when the anticorrosive current anode disappears due to abnormal consumption or dropping, severe corrosion may proceed immediately.

In addition to the above technology, for example, a technique such as Patent Document 1 (Japanese Patent No. 2001-17381) has been proposed to improve the corrosion resistance of the steel material itself. This technique is excellent in corrosion resistance by adjusting the chemical composition of steel materials suitably, and the corrosion resistance steel for shipbuilding which can be used even without coating is disclosed. In addition, Patent Document 2 (Japanese Patent Laid-Open No. 2002-26605) discloses a marine steel material in which the life of the coating film is improved by appropriately adjusting the chemical composition of the steel material. These techniques are known to be able to secure a certain degree of corrosion resistance compared with the prior art.

However, under severe corrosive environments, the corrosion resistance is still not sufficient, so the corrosion resistance is required again. In particular, corrosion (so-called crevice corrosion) in the contact areas between foreign matter and steel, structural reasons or damage areas of anticorrosive coatings, etc., may occur remarkably, which may reduce the life. The proposed technique has insufficient corrosion resistance in this area.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its object is to improve durability against shipbuilding steel, especially crevice corrosion, which is excellent in corrosion resistance that can be put into practical use without the use of coating or electric method. The present invention provides a shipbuilding steel material that exhibits excellent durability against salt adhesion and corrosion caused by a wet environment.

(Means to solve the task)

As a steel material for shipbuilding of the present invention for achieving the above object, in mass%,

C: 0.01 ~ 0.30%

Si: 0.01 ~ 1.50%

Mn: 0.01 to 2.0%

Al: in addition to containing 0.005 to 0.10%,

Co: 0.01 ~ 5.00% and

Mg: contains 0.0005 to 0.020%,

The point is that the balance consists of Fe and unavoidable impurities. In such ship steel materials, it is preferable to adjust the value ([Co] / [Mg]) of the ratio of Co content [Co] and Mg content [Mg] to the range of 2-350.

In the shipbuilding steel of the present invention, if necessary,

(1) Cu: 0.01-5.0%,

    Cr: 0.01 ~ 5.0%

    Ni: 0.01-5.0% and

    Ti: at least one selected from the group consisting of 0.005 to 0.20%,

(2) Ca: 0.0005 to 0.020%,

(3) Mo: 0.01 to 5.0% and / or

    W: 0.01 to 2.0%,

(4) B: 0.0001 ~ 0.010%

    V: 0.01 to 0.50% and

    Nb: one or more selected from the group consisting of 0.003 to 0.50%,

(5) Zn: 0.001 to 0.10% and the like were also effective, and the properties of shipbuilding steels were also improved according to the type of the contained components.

(The best form to carry out invention)

MEANS TO SOLVE THE PROBLEM The present inventors earnestly researched in order to solve the said subject. As a result, when a certain amount of Co and Mg were used in combination and the chemical composition was properly adjusted, it was found that shipbuilding steels capable of solving the above problems could be realized, and thus the present invention was completed.

It is important that the steel materials of the present invention are used in combination with Co and Mg, and even if any of these components is omitted, the object of the present invention cannot be achieved. Although the effect of each of these components will be mentioned later, the reason why corrosion resistance improved by using these together can be considered as follows.

Mg is to suppress the pH decrease in the corrosion portion and to suppress the corrosion reaction to improve the corrosion resistance. This is because in the component system of ordinary steel (for example, Si-Mn steel), since the rust generated is porous, the molten Mg does not remain near the surface of the steel sheet and is immediately external (for example, in seawater). It will spread to. Therefore, containing Mg alone reduces the effect of improving corrosion resistance. However, by containing Co like Mg, a fine surface rust film is formed and diffusion of Mg to the outside is suppressed. It has also been found that the synergistic effect of the dissolved Co with the hydrolysis average reaction can significantly improve the corrosion resistance.

Although such an effect is exhibited by being controlled by the appropriate amount mentioned later, it is preferable to also control suitably the value ([Co] / [Mg]: mass ratio) of these content ratios. That is, when this value ([Co] / [Mg]) is less than 2, it is easy to suppress local corrosion, and when it exceeds 350, suppression of whole surface corrosion becomes inadequate. The value of [Co] / [Mg] is preferably about 10 to 350, more preferably about 20 to 60.

In order for the steel of this invention to satisfy the basic characteristics as the steel, it is necessary to adjust basic components, such as C, Si, Mn, and Al suitably. The reason for the range limitation of these components is shown next together with the effect by each element of said Co and Mg.

C: 0.01 ~ 0.30%

C is an element necessary for securing the strength of the material. In order to obtain the minimum strength as the structural member of the ship, that is, about 400 MPa (depending on the thickness of the steel used), it is necessary to contain 0.01% or more. However, when it contains exceeding 0.30%, toughness will deteriorate. Therefore, the content range of C is made 0.01 to 0.30%. Moreover, the minimum with preferable C content is 0.02%, More preferably, it is good to set it as 0.04% or more. Moreover, the upper limit with preferable C content is 0.28%, More preferably, it is good to set it as 0.26% or less.

Si: 0.01 ~ 1.50%

Si is an element necessary for deoxidation and securing strength, and if it does not satisfy 0.01%, it cannot secure the minimum strength as a structural member. However, when it contains exceeding 1.50% excessively, weldability will deteriorate. Moreover, the minimum with preferable Si content is 0.02%, More preferably, it is good to set it as 0.15% or more. In addition, the upper limit with preferable Si content is 1.25%, More preferably, it is good to set it as 1.00% or less.

Mn: 0.01 to 2.0%

Mn, like Si, is an element necessary for deoxidation and securing strength, and if it does not satisfy 0.01%, it cannot secure the lowest strength. However, when it contains exceeding 2.0% excessively, toughness will deteriorate. Moreover, the minimum with preferable Mn content is 0.05%, More preferably, it is good to set it as 0.10% or more. In addition, the upper limit with preferable Mn content is 1.80%, More preferably, it is good to set it as 1.60% or less.

Al: 0.005 ~ 0.10%

Al, like Si and Mn, is an element necessary for deoxidation and securing strength. If Al is not satisfied with 0.005%, Al is not effective in deoxidation. However, since the weldability deteriorates when it exceeds 0.10%, the range of Al addition amount was made into 0.005 to 0.10%. Moreover, the minimum with preferable Al content is 0.010%, More preferably, it is good to set it as 0.015% or more. In addition, the upper limit with preferable Al content is 0.040%, More preferably, it is good to set it as 0.050% or less.

Co: 0.01 ~ 5.0%

被膜)을 형성하는 데 필요불가결한 원소이다. 이러한 효과를 발휘시키기 위해서는, Co 함유량을 0.01% 이상으로 하는 것이 필요하다. 그렇지만, 5.0%를 초과하여 과잉 함유시키면 용접성이 열화한다. 따라서, Co 함유량은 0.01~5.0%로 하였다. 또한, Co 함유량의 바람직한 하한은 0.015%이고, 보다 바람직하게는 0.020% 이상으로 하는 것이 좋다. 또한, Co 함유량의 바람직한 상한은 4.5%이고, 보다 바람직하게는 4.0% 이하로 하는 것이 좋다.Co is a dense surface rust coating that contributes greatly to the corrosion resistance of steels in high salt environments.

Iii) an indispensable element for forming. In order to exhibit such an effect, it is necessary to make Co content into 0.01% or more. However, when it contains exceeding 5.0%, weldability will deteriorate. Therefore, Co content was made into 0.01 to 5.0%. Moreover, the minimum with preferable Co content is 0.015%, More preferably, it is good to set it as 0.020% or more. Moreover, the upper limit with preferable Co content is 4.5%, More preferably, it is good to set it as 4.0% or less.

Mg: 0.0005 ~ 0.020%

Since Mg exhibits a pH synergism by dissolution, it has the effect of suppressing the pH decrease due to hydrolysis reaction in the local anode where iron dissolution occurs, suppressing the corrosion reaction and improving corrosion resistance. It is necessary to contain Mg 0.0005% or more in order to exhibit such an effect, but when it contains more than 0.020%, workability and weldability will deteriorate. Therefore, the Mg content is appropriately in the range of 0.0005 to 0.020%. The minimum with preferable Mg content is 0.0007%, More preferably, it is good to contain 0.0010% or more. Moreover, the upper limit with preferable Mg content is 0.018%, More preferably, it is good to set it as 0.015% or less.

The basic component in the ship steel of this invention is as above-mentioned, and remainder consists of iron (Fe) and an unavoidable impurity (for example, P, S, O, etc.). Besides these, components (for example, Zr, N, etc.) of the grade which do not impair the characteristic of steel materials are also acceptable. However, these allowable components deteriorate toughness when their amount is excessive, and should be suppressed to about 0.1% or less.

In addition to the above components, the marine steel material of the present invention may optionally contain one or more selected from the group consisting of (1) Cu, Ni, Ti, and Cr, (2) Ca, (3) Mo and / or W, ( 4) One or more selected from the group consisting of B, V, and Nb, and (5) Zn and the like are also effective, and the properties of shipbuilding steels are also improved depending on the type of the contained components.

At least one member selected from the group consisting of Cu: 0.01 to 5.0%, Cr: 0.01 to 5.0%, Ni: 0.01 to 5.0%, and Ti: 0.005 to 0.20%

Cu, Cr, Ni, and Ti are all effective elements for improving corrosion resistance. Among them, Cu and Cr, like Co, are effective elements for forming a dense surface rust coating which greatly contributes to the improvement of corrosion resistance. In order to exhibit such an effect, it is preferable to contain all 0.01% or more, but when it contains too much, weldability and hot workability will deteriorate, It is preferable to set it as 5.00% or less. The minimum with more preferable at the time of containing Cu and Cr is 0.05%, and a more preferable upper limit is 4.50%.

Ni is an effective element that greatly contributes to the improvement of corrosion resistance and stabilizes a dense surface rust coating. In order to exert such an effect, it is preferable to contain Ni by 0.01% or more. However, when the content of Ni is excessive, the weldability and hot workability deteriorate, so it is preferable to set it to 5.0% or less. The minimum with more preferable at the time of containing Ni is 0.05%, and a more preferable upper limit is 4.50%.

Ti is an element that greatly contributes to the improvement of corrosion resistance, densifies the surface rust coating, improves its environmental barrier properties, and inhibits corrosion in the gap, thereby improving the corrosion resistance of the gap. In order to ensure the corrosion resistance required under such an environment, it is preferable to contain 0.005% or more, but excessively containing more than 0.20% deteriorates workability and weldability. The minimum with more preferable Ti containing is 0.008%, and a more preferable upper limit is 0.15%.

Ca: 0.0005 ~ 0.020%

Ca, like Mg, is an element that exhibits a pH synergism by dissolution, and is effective in improving the corrosion resistance by inhibiting the pH decrease due to hydrolysis reaction and inhibiting the corrosion reaction in the local anode where iron dissolution occurs. . Such an effect by Ca is effectively exhibited by containing 0.0005% or more, but excessively containing more than 0.020% deteriorates workability and weldability. The minimum with more preferable Ca containing is 0.0010%, and a more preferable upper limit is 0.015%.

Mo: 0.01% to 5.0% and / or W: 0.01% to 2.0%

Mo and W have the effect of increasing the uniformity of corrosion to suppress the generation of holes due to local corrosion. In particular, by containing simultaneously with Co, a remarkable homogeneous corrosive improvement effect is exhibited. In order to exhibit such an effect, it is preferable to contain all 0.01% or more, but when it contains excessively, weldability will deteriorate, It is preferable to make Mo 5.0% or less and W 2.0% or less. The minimum with more preferable Mo when it contains is 0.02%, and a more preferable upper limit is 4.50%. Moreover, the minimum with more preferable at the time of containing W is 0.02%, and a more preferable upper limit is 1.8%.

At least one selected from the group consisting of B: 0.0001 to 0.010%, V: 0.01 to 0.50%, and Nb: 0.003 to 0.50%

There are cases where high strength is required for ship steels depending on the part to be applied, and these elements are necessary for improving the strength. Among them, B is contained in an amount of 0.0001% or more, which improves hardenability and is effective in improving strength. However, when B is contained in excess of 0.010%, the base metal toughness deteriorates. V is effective in improving strength by containing 0.01% or more, but excessively exceeding 0.50% causes deterioration of toughness of the steel, which is not preferable. Nb is contained in an amount of 0.003% or more, which is effective in improving strength. However, when Nb is contained in excess of 0.50%, toughness of the steel is caused. Moreover, the minimum with more preferable of these elements is 0.0003% for B, 0.02% for V, and 0.005% for Nb. The upper limit of B is more preferably 0.0090%, V is 0.45%, and Nb is 0.45%.

Zn: 0.01 ~ 0.10%

Zn reacts with salt or sulfur to form zinc chloride or zinc sulfide on the surface of the steel, and inhibits corrosion by blocking steel from water from the environment. Zinc chloride and zinc sulfide are easily deposited on the surface of the steel without flowing out to the sea in the gap between the coating film and the gap where the material movement is limited, and therefore, the corrosion inhibitory effect is particularly high at the bottom of the film or in the gap.

In order to achieve such an effect and to ensure required corrosion resistance, the content of Zn needs to be 0.001% or more. However, when it contains exceeding 0.10% excessively, workability and weldability will deteriorate. Therefore, content of Zn is made into 0.001 to 0.10%. Moreover, the minimum with preferable Zn content is 0.003%, More preferably, it is good to set it as 0.005% or more. In addition, the upper limit with preferable Zn content is 0.09%, More preferably, it is good to set it as 0.08% or less.

In the case of welding the steel of the present invention to form a welded structure, when welding is carried out under normal welding conditions or using a material, there is a case where corrosion resistance is not expressed in the welded part because the concentration of the effective element changes in the welded joint. In particular, the contents of Mg and Co must satisfy the following formulas (1) and (2), respectively.

0.30 ≤ Co content of deposited metal / Co content of base metal ≤ 3.0 ‥‥ (1)

0.30 ≤ Mg content of deposited metal / Mg content of base metal ≤ 3.0 ‥‥ (2)

In other words, when the ratio between the weld metal and the base metal (content of the weld metal / content of the base metal) does not exceed 0.30 (less than 0.30), the synergistic effect of improving the corrosion resistance by the addition of these elements is obtained. It is not expressed and the corrosion resistance of the weld metal part becomes insufficient. In addition, when this ratio exceeds 3.0, the toughness of the welded portion will occur, which is not preferable in view of mechanical strength. Therefore, it is recommended that the ratio be adjusted to 0.30 to 3.0, more preferably 0.5 to 2.0.

Moreover, also about Cu, Cr, Ni, Ti, Ca, Mo, W, and Zn which are elements effective for improving corrosion resistance other than Mg and Co, when adding these, ratio of content of a weld metal and a base material (content of a weld metal / It is recommended to adjust the content of the base metal) to 0.30 to 3.0, and more preferably to 0.5 to 2.0.

The steel for shipbuilding of the present invention basically aims to exhibit excellent corrosion resistance of the steel itself without coating, but if necessary, the tar epoxy resin coating material or other representative heavy methods shown in Examples described later. It is also possible to use it in combination with other methods such as painting, heavy paint, shot primer, and electric. In the case of performing such anticorrosive coating, the corrosion resistance (coating corrosion resistance) of the coating film itself is also good as shown in Examples described later.

Hereinafter, an Example is given and this invention is demonstrated more concretely. The present invention is not limited to the following examples in the original, and of course, the present invention can be modified as appropriate within the range suitable for the above-described purpose, and all of them are included in the technical scope of the present invention.

(Example)

Example 1

Steels having a chemical composition range of the present invention shown in Tables 1 to 3 were dissolved in a rotary furnace, and various steel sheets were manufactured by continuous casting and hot rolling. The obtained steel sheet was cut and surface-grinded, and finally, the test piece of 100x100x25 (mm) size was produced (test piece A). The external shape of the test piece A is shown in FIG.

TABLE 1

TABLE 2

TABLE 3

As shown in Fig. 2, four small test pieces of 20 × 20 × 5 (mm) were brought into contact with a large test piece of 100 × 100 × 25 (mm) (same as the above test piece A) to form a gap. B was produced. The small test piece and the large test piece for gap formation are steel materials of the same chemical composition, and the surface finish was also surface-grinded like the test piece A. Then, a 5 mm phi hole was drilled in the center of the small test piece, and a screw hole was drilled in the base material side (large test piece side) and fixed with a screw made of M4 plastic.

Moreover, the test piece C (FIG. 3) which carried out the tar epoxy resin coating of the average thickness of 250 micrometers (the lower surface: Zn-rich primer) on the whole surface was also used. In order to investigate the degree of corrosion progress when the damaged steel is exposed to the coating film for the anticorrosion method, a cut (length: 100 mm, width: about 0.5 mm) extending to the base was formed on one side of the specimen C. .

About specimens of each chemical composition of the present invention shown in Tables 1 to 3, five specimens A, specimen B and specimen C were used for the corrosion test. The corrosion test method at this time is as follows.

Corrosion Test Method

First, the marine environment was simulated, and the combined cycle corrosion test was repeated by repeating the sea water spray test and the constant temperature and humidity test. In the sea water spray test, the test specimens (each specimens A to C) were installed in the test chamber at an angle of 60 ° from the horizontal, and artificial seawater (saline) at 35 ° C. was sprayed in a mist state. The spraying of the brine was always carried out continuously. At this time, the spray amount was adjusted in advance so that 1.5 ± 0.3 mL of artificial seawater was collected at an arbitrary position in a circular dish having an area of 80 cm 2, which was installed horizontally in the test tube. The constant temperature and humidity test was carried out in a test tube adjusted to a temperature of 60 ° C. and a humidity of 95% by installing the test material at an angle of 60 ° from the horizontal. Sea water spray test: 4 hours, constant temperature and humidity test: 4 hours were made into 1 cycle, and this was done alternately, and the test material was corroded. The total test time was 6 months.

(1) The test piece A converted the weight change before and after the test into the average plate thickness reduction amount D-ave (mm), calculated the average value of five test pieces, and evaluated the front corrosion of each specimen. In addition, the maximum erosion depth D-max (mm) of the specimen A was obtained using a tactile three-dimensional shape measuring device, and normalized to an average plate thickness reduction amount [D-ave (mm)] (that is, Calculation of D-max / D-ave] was used to evaluate the corrosion uniformity, and the gravimetric and plate thickness measurements after the test were performed by corrosion of rust and the like by cathodic electrolysis [JIS K8284] in aqueous hydrogen citrate diammonium solution. The product was removed and run.

(2) Test piece B performs visual observation of crevice corrosion (contact surface), investigates the presence of crevice corrosion, and if crevice corrosion appears, removes corrosion products by the cathodic electrolysis method. Depth D-crev (mm) of maximum crevice corrosion was measured using a dimensional shape measuring device.

(3) The test piece C (scratched) which the coating process was performed measured the ratio (expansion area ratio) of the area which the coating film expanded to the surface where the scratch was formed after a test. The expansion area ratio was calculated by the lattice point method (lattice spacing 1mm). That is, the number of lattice points for which expansion was confirmed was subtracted from all lattice points, and the average value of five test pieces was obtained by defining it as the expansion area ratio. In addition, the width | variety which the coating film of the direction perpendicular | vertical to the flaw expanded was measured by vernier, and the maximum value of five test pieces was defined as the maximum expansion width.

The above-mentioned corrosion resistance (D-ave), corrosion uniformity (D-max / D-ave), crevice corrosion resistance (D-crev), coating corrosion resistance (expansion area ratio and maximum expansion width) Evaluation criteria were as Table 4 below. Corrosion test results are shown in the following Tables 5-6.

TABLE 4

TABLE 5

TABLE 6

From these results, it can consider as follows. Nos. 2 and 3 which do not contain either Co or Mg, and Nos. 4 and No. 5 in which the content of Co or Mg does not satisfy the lower limit specified in the present invention are those of Co or Mg. By the addition effect, the front corrosion resistance slightly improved compared with the conventional steel (No. 1). However, in No. 2 which does not contain Co and No. 4 in which Co amount is insufficient, there is no improvement effect in corrosion uniformity and expansion area ratio. In addition, in No. 3 that does not contain Mg and No. 5 in which the Mg amount is insufficient, the improvement effect is not exhibited in the crevice corrosion resistance and the maximum expansion width, and thus, the corrosion resistance of ship steel is insufficient.

On the other hand, those containing Co and Mg in combination with appropriate amounts (Nos. 6 to 50) are all synergistic by the addition of these elements, which are also superior in corrosion resistance to conventional steels (No. 1), and are corrosion resistant for ships. As a result, the result was obtained. In particular, it has been found that the corrosion resistance of steel materials is further improved by including corrosion resistance improving elements such as Cu, Cr, Ni, Ti, Ca, Mo, W and Zn in addition to the use of Co and Mg.

Among these, the test material to which Cu, Cr, Ni, or Ti was added, especially the effect of reducing the largest expansion width of a coating test material (No.13-No.15 etc.) is recognized, and the rust densification of these elements is the rust of a cut part. It is presumed to have suppressed the development of corrosion by acting on stabilization. In addition, the effect of improving the corrosion resistance of Ca is recognized (No. 16, No. 20, No. 22, etc.), and Ca can be considered to have reduced corrosion by further strengthening the inhibition of pH decrease in the gap. . In addition, it can be seen that the addition of Mo or W is very effective in improving the corrosion uniformity and coating expandability (Nos. 31 to 33). In addition, as can be seen from the results of No. 30, No. 33, No. 34, No. 35, etc., by appropriately adjusting the value of ([Co] / [Mg]), various corrosion resistances are obtained. It became. In addition, Zn-added test materials (No. 40 to No. 42, etc.) have further improved coating corrosion resistance and crevice corrosion resistance. For example, No. 41, in which a proper amount of Zn was added to Mg and Co in combination, resulted in a reduction in the expansion area ratio of Test Piece C compared to No. 6 in which only Mg and Co were used in combination. The improvement of corrosion resistance by the addition of Zn is believed to be a result of the effect of inhibiting corrosion by blocking the possession of steel from moisture from the surrounding environment by forming a deposition film of zinc chloride or zinc sulfide on the surface of the steel.

Example 2

The steels of the chemical composition shown in Table 7 were then dissolved in a rotary furnace, and various steel sheets were produced by continuous casting and hot rolling. The obtained steel sheet was cut and surface-grinded, and finally, the test piece D 'of 300x150x25 (mm) size was produced. The sub-marge arc welding was performed using the welding material of the chemical component shown in Table 8, and the seam test piece D shown in the figure was produced rather than D'2 (FIG. 4). In addition, the wire diameter of all the welding materials was 4.8 mm, and the improved shape was V-shaped. The amount of heat input was appropriately adjusted from 1 to 10 kJ / mm.

Further, two small test pieces of 60 × 60 × 5 (mm) were brought into contact with the weld portion of the test piece D to prepare a test piece E having a gap portion (FIG. 5). The chemical composition of the small test piece for gap formation was the same as that of the test piece D, and the surface finish was subjected to the same surface grinding as the test piece D. And the hole of 10 mm (phi) was made to the center of a small test piece, the screw hole was made to the base material side (large test piece side), and it fixed with the screw made of M8 plastic.

Moreover, the test piece F (FIG. 6) which carried out the tar epoxy resin coating of the average thickness of 250 micrometers (the lower degree: Zn-rich primer) on the whole surface was also used. In order to investigate the degree of corrosion progress when the base steel which has scratched the coating film for corrosion protection is exposed, a flaw (length: 200 mm, width: about 0.5 mm) extending to the base is perpendicular to the welding line on one side of the specimen F. It was formed by a cut knife in the horizontal direction.

TABLE 7

TABLE 8

The seam test specimens prepared using the base materials and the welding materials shown in Tables 7 and 8 were subjected to a corrosion test using five test specimens D, five test specimens, and five test specimens, respectively. At this time, the corrosion test method (solid line exposure test) was as follows.

Corrosion Test Method

Five test specimens and five test specimens D to F were installed on the bottom plate of the tank inner surface of the VLCC tanker, and the corrosion status of each specimen was examined after normal operation for five years. After 5 years of exposure, the specimen D was removed from corrosion products such as iron rust by the cathodic electrolytic method [JIS K8284] in aqueous hydrogen citrate 2 ammonium solution. In addition, also about the test piece E, the small test piece for gap formation was removed, and the corrosion product was removed by the same method.

(1) The test piece D converted the weight change before and after the test into the average plate | board thickness reduction amount D-ave (mm), calculated the average value of 5 test pieces, and evaluated the front corrosion of each test material. In addition, the maximum erosion depth D-max (mm) of the test piece D was obtained using a tactile three-dimensional shape measuring device, and normalized to an average plate thickness reduction amount [D-ave (mm)] (that is, D-max / D). -ave was calculated) to evaluate the corrosion uniformity.

(2) The test piece E measured the maximum crevice corrosion depth D-crev (mm) of the big test piece side using the stylus three-dimensional shape measuring apparatus.

(3) The test piece F (cut scratch) subjected to the coating treatment measured the coating film expansion width (mm) in a direction perpendicular to the cut scratch, and defined the maximum value of the five test specimens as the maximum expansion width.

The front corrosion resistance (average plate reduction amount: D-ave), corrosion uniformity (D-max / D-ave), crevice corrosion resistance (D-crev), coating corrosion resistance (maximum expansion width) Evaluation criteria were shown in Table 9. The corrosion test results are shown in Table 10 below. In Table 10, I (Co) represents Co content of the weld metal / Co content of the base metal, I (Mg) represents the Mg content of the weld metal / Mg content of the base metal.

TABLE 9

TABLE 10

No. 51 and No. 52, in which the Mg and Co content does not satisfy the relation (1) or (2) in the weld, are excellent in front corrosion resistance but satisfactory in other corrosion characteristics such as crevice corrosion resistance. No results were obtained. This is the result of the progress of corrosion in the part of the weld metal. On the other hand, in Nos. 53 to No. 58 conventional corrosion resistant steels (Nos. 2 to 4) in which the above ratios satisfy the relations (1) and (2), the corrosion resistance was improved even in the corrosion characteristics, and the welding The corrosion resistance preferable as a structure was shown.

In addition, in this embodiment, although the welding part by the submagazine arc welding method was made into evaluation object, the same effect can be acquired by other welding methods, such as a covering arc welding method and an electroslag welding method. In addition, the welding material used is also not limited to Table 8.

The shipbuilding steels of the present invention contain a predetermined amount of Co and Mg in combination, and by appropriately adjusting the chemical composition, shipbuilding steel with excellent corrosion resistance that can be put into practical use without coating and electrical methods can be realized. In addition to improving durability against crevice corrosion, shipbuilding steels that exhibit excellent durability against salt adhesion due to seawater and corrosion due to wet environments can be realized. Such shipbuilding steels are useful as materials for tankers, cargo ships, passenger ships, passenger ships, warships, etc. of outer shells, ballast tanks, crude oil tanks, and the like.

Claims (8)

C: 0.01 to 0.30% (mass%, the same below), Si: 0.01-1.50%, Mn: 0.01 to 2.0%, Al: 0.005 to 0.10%, respectively, in addition to A ship steel having excellent corrosion resistance, characterized by containing 0.01% to 5.00% of Co and 0.0005% to 0.020% of Mg, and the balance being made of iron (Fe) and unavoidable impurities. The method of claim 1, The value ([Co] / [Mg]) of ratio of said Co content [Co] and Mg content [Mg] is 2-350, The ship steel material excellent in corrosion resistance. The method according to claim 1 or 2, Also, Cu: 0.01-5.0%, Cr: 0.01 to 5.0%, A ship steel having excellent corrosion resistance, characterized by containing at least one member selected from the group consisting of Ni: 0.01 to 5.0% and Ti: 0.005 to 0.20%. The method according to any one of claims 1 to 3, Also, Ca: ship steel materials excellent in corrosion resistance, containing 0.0005 to 0.020%. The method according to any one of claims 1 to 4, Also, A ship steel having excellent corrosion resistance, characterized by containing Mo: 0.01 to 5.0% and / or W: 0.01 to 2.0%. The method according to any one of claims 1 to 5, Also, B: 0.0001 ~ 0.010% A ship steel having excellent corrosion resistance, characterized by containing at least one member selected from the group consisting of V: 0.01 to 0.50% and Nb: 0.003 to 0.50%. The method according to any one of claims 1 to 6, Also, Zn: ship steel having excellent corrosion resistance, characterized by containing 0.001 to 0.10 mass%. The relationship between Co and Mg contained in the weld metal of a welded structure formed by welding the ship steel materials described in any one of Claims 1-7, and a base steel material is A marine welded structure having excellent corrosion resistance, characterized by satisfying (1) and (2). 0.30 ≤ Co content of deposited metal / Co content of base metal ≤ 3.0 ‥‥ (1) 0.30 ≤ Mg content of deposited metal / Mg content of base metal ≤ 3.0 ‥‥ (2)

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